1. Field
The present invention relates to a motor speed control integrated circuit.
2. Description of the Related Art
Various electronic apparatuses have an exothermic body that generates heat when the electronic apparatus operates. To cool this exothermic body, a fan motor is usually provided. For example, in personal computers, servers and the like, the operating frequencies of the CPUs become increasingly higher causing the heat values of the CPUs to increase. Accordingly, a fan motor for cooling the CPU and a motor driver to drive the fan motor is usually provided in personal computers, servers and the like.
As a speed control method for fan motors, a speed servo control method has been proposed which has a PWM drive method combined therewith as shown, e.g., in FIG. 10 (refer, for example, to Japanese Patent Application Laid-Open Publication No. 2003-204692). To describe in detail, a speed specifying circuit 3 generates and outputs a speed-specifying signal to instruct to accelerate/decelerate based on a rotational speed detection signal obtained from a pulse generator PG of a motor 1 and on a number-of-rotations setting signal from an external device such as a microcomputer and a DSP, and has the speed-specifying signal integrated by an integrator 5. A comparator 9 compares a triangular-wave signal generated by a triangular-wave generator 7 and the speed-specifying signal integrated by the integrator 5 and generates a PWM (Pulse Width Modulation) signal as the comparing result. A motor driver 11 controls the rotational speed of the motor 1 on the basis of the PWM signal from the comparator 9 by causing a corresponding amount of current to the duty ratio of the PWM signal to flow through a drive coil of the motor 1. Furthermore, a hall element 13 is provided for the stator of the motor 1, and the motor driver 11 controls the rotational direction of the motor 1 by switching the direction of the current flowing through the drive coil on the basis of the hall element output of the hall element 13 indicating the detected position of the rotor.
As another speed control method for fan motors, a technique has been proposed which use a thermistor as shown, e.g., in FIG. 11 (refer, for example, to Japanese Patent Application Laid-Open Publication No. 2005-80500). To describe in detail, when an electronic apparatus operates with a heavy load and thus temperature increases, the cooling fan motor operates at full speed, but even when the load switches to being light and temperature decreases, the fan motor usually continues to operate at full speed thereby consuming power wastefully. Accordingly, a thermistor RTH is provided to monitor temperature variation, and a divided voltage VTH, produced by division of a supply voltage between a resistor R and the thermistor RTH, is adjusted according to the temperature variation. The motor driver 20 compares the divided voltage VTH and a triangular-wave signal (not shown) and generates a PWM signal as the comparing result and outputs to a transistor TR. As a result, the ON/OFF periods of the transistor TR change according to the duty ratio of the PWM signal from the motor driver 20. The change in the ON/OFF periods of the transistor TR adjusts the amount of current flowing through the drive coil of the fan motor, thereby controlling the rotational speed of the fan motor. For example, when the temperature of the electronic apparatus increases due to its heavy load, the resistance of the thermistor RTH and the divided voltage VTH drop thereby increasing the rotational speed of the fan motor. On the other hand, when the temperature of the electronic apparatus decreases due to its light load, the resistance of the thermistor RTH and the divided voltage VTH rise thereby decreasing the rotational speed of the fan motor.
As shown in FIGS. 10 and 11, various methods for the speed control of the fan motor can be adopted depending on the application of electronic apparatuses. Accordingly, logics other than the motor driver for driving a motor are implemented by farm ware incorporated in a general-purpose microcomputer, or circuitry consisting of discretes, or the like, and changes in design need to be made depending on their specifications, which is particularly troublesome.